<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .active {
            color: red;
        }
    </style>
</head>
<body>

<div id="app">
    <ul>
        <li v-for="(m,i) in movies" v-on:click="change(i)" :class="getClasses(i)">{{i}} - {{m}}</li>
    </ul>
</div>

<script src="../js/vue.js"></script>
<script>
    const app = new Vue({
        el: '#app',
        data: {
            movies: ['a', 'b', 'c', 'd'],
            activeEle: 0,
        },
        methods: {
            change: function (i) {
                this.activeEle = i;
            },
            getClasses: function (i) {
                if (i == this.activeEle) {
                    return 'active';
                } else {
                    return '';
                }
            }
        }
    })
</script>

</body>
</html>